1. Field of the Invention
Generally, the present disclosure relates to the manufacture of sophisticated semiconductor devices, and, more specifically, to a pellicle for use during extreme ultraviolet (EUV) photolithography processes with an aerogel support frame.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPU's, storage devices, ASIC's (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout, wherein field effect transistors (NMOS and PMOS transistors) represent one important type of circuit element used in manufacturing such integrated circuit devices. In general, integrated circuit devices are formed by performing a number of process operations in a detailed sequence or process flow. Such process operations typically include deposition, etching, ion implantation, photolithography and heating processes that are performed in a very detailed sequence to produce the final device.
Device designers are under constant pressure to increase the operating speed and electrical performance of transistors and integrated circuit products that employ such transistors. One technique that continues to be employed to achieve such results is the reduction in size of the various devices, such as the gate length of the transistors. The gate length (the distance between the source and drain regions) on modern transistor devices may be approximately 30-50 nm, and further downward scaling is anticipated in the future. Manufacturing devices that are so small is a very difficult challenge, particularly for some processes, such as photolithography tools and techniques.
Known photolithography tools include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Photolithography tools and systems typically include a source of radiation at a desired wavelength, an optical system and, typically, the use of a so-called mask or reticle that contains a pattern that is desired to be formed on a wafer. Radiation is provided through or reflected off the mask or reticle to form an image on a semiconductor wafer. The radiation used in such systems can be light, such as ultraviolet light, deep ultraviolet light (DUV), vacuum ultraviolet light (VUV), extreme ultraviolet light (EUV), etc. The radiation can also be x-ray radiation, e-beam radiation, etc. Generally, the image on the reticle is utilized to irradiate a light-sensitive layer of material, such as photoresist material. Ultimately, the irradiated layer of photoresist material is developed to define a patterned mask layer using known techniques. The patterned mask layer can be utilized to define doping regions, etched structures associated with an integrated circuit, etc. Currently, most of the photolithography systems employed are so-called deep ultraviolet systems (DUV) that generate radiation at a wavelength of 248 nm or 193 nm. However, the capabilities and limits of traditional DUV photolithography systems are being tested as device dimensions continue to shrink. This has led to the development of a so-called EUV system that uses radiation with a wavelength less than 20 nm, e.g., 13.5 nm.
Reducing particle contamination in photolithography processes, particularly on the reticle, has always been an ongoing issue that must be addressed. The presence of even very minute particles on the reticle or the associated equipment or wafer during the photolithography process may lead to the patterning of inaccurate or undesirable features on a wafer, and may lead to the formation of devices with reduced performance capabilities. In many cases, the presence of undesirable particles during photolithography processes may render the resulting devices inoperable. For that reason, semiconductor manufacturers go to great lengths and great expense to keep the photolithography processes they employ as clean as possible. This involves very detailed and expensive handling and cleaning procedures for all of the components of a photolithography system, including the reticles. The cleanliness requirement for photolithography processes is only going to increase as EUV systems are adopted because the EUV systems are sensitive to contamination by extremely small particles that might not create a problem for DUV systems. In addition, other non-particulate forms of contamination, e.g., organic and inorganic chemical contaminants, even at the level of a few atomic layers, must be prevented from adhering to critical surfaces.
Most modern photolithography tools include a pellicle that is positioned between the reticle and the wafer. Generally, conventional DUV photolithography systems which utilize wavelengths of 193 nm or more include the pellicle to seal off the mask or reticle to protect it from airborne particles and other forms of contamination. As mentioned above, contamination on the surface of the reticle or mask can cause manufacturing defects on the wafer. For example, pellicles are typically used to reduce the likelihood that particles might migrate into an exposure field of a reticle in a stepping lithographic system, i.e., into the object plane of the imaging system. If the reticle or mask is left unprotected, the contamination can require the mask or reticle to be cleaned or discarded. Cleaning the reticle or mask interrupts valuable manufacturing time and discarding the reticle or mask is costly. Replacing the reticle or mask also interrupts valuable manufacturing time.
A pellicle typically includes a pellicle frame and a membrane. The pellicle frame may include one or more walls which are securely attached to the absorber (chrome) side of the mask or reticle for an EUV application. Pellicles have also been employed with anti-reflective coatings on the membrane material. The membrane is stretched across the metal frame and is employed in an effort to prevent any contaminants from reaching the mask or reticle. The membrane is preferably thin enough to avoid the introduction of aberrations and to be optically transparent and yet strong enough to be stretched across the frame. The optical transmission losses associated with the membrane of the pellicle can affect the exposure time and throughput of the photolithography system. The optical transmission losses are due to reflection, absorption and scattering. Stretching the membrane ensures that it is substantially flat and does not adversely affect the image projected onto the wafer. The membrane of the pellicle generally covers the entire printable area of a mask or reticle and is sufficiently durable to withstand cleaning and handling.
Pellicles for EUV systems should be stable enough to retain their shape over long periods of time and many exposures to flashes of radiation and be tolerant of repeated maintenance procedures. Small particles that adhere to the pellicle surface (the membrane) generally do not significantly obstruct light directed to the surface of the wafer. The metal frame ensures that a minimum stand-off distance from the mask is provided to ensure that no more than about a 10% reduction in light intensity on the wafer surface is achieved for a particle of a particular size. The pellicle also tends to keep any optical signatures due to particles out of the depth of field of the lens. Thus, the stand-off distance of the pellicle is important in preventing contaminants from being imaged onto the wafer since the depth-of-field of the imaging lens is orders of magnitude smaller than the pellicle-mask stand-off distance.
Conventional materials used as a pellicle for EUV lithographic systems include thin metallic or ceramic films stretched and mounted over the reticle. Such films have usually consisted of membranes formed from silicon, silicon nitride, synthetic diamond, diamond-like carbon, etc. To avoid a huge loss of light throughput due to material absorption, these membranes typically have a maximum thickness in the range of about 50-100 nm. These membranes typically cover a relatively large area of about 100-200 cm2. At such small thicknesses, these membranes are prone to destruction due to mechanical loading (from mounting and vibrations) and thermo-mechanical loading due to heat-induced stress. The heating effect is a direct result of the intrinsically high absorption of all substances in the EUV spectral region of interest (around 13.5 nm). Furthermore, the thermal loading at incident optical powers approaching several watts of in-band EUV power (likely needed for high volume manufacturing) can severely deform and even melt the membranes. Some attempts to counteract these mechanical shortcomings have been made by mounting the membranes on a rigid wire mesh. See, e.g., Schroff et. al., “High transmission pellicles for extreme ultraviolet lithography reticle protection,” J. Vac. Sci. Technol., B28, C6E36 (2010). However, such a solution has proven to be unworkable, probably due to the high light loss and light scattering as a result of the wire mesh backbone of the membrane. Such an approach has been largely abandoned.
There is a need for a pellicle to be used in EUV applications that is more durable or stable than conventional pellicle materials. The present invention is directed to several different embodiments of such a pellicle.